I am in the examples folder trying to run 
   
   - Electromagnetism in Minkowski spacetime 
   
<https://nbviewer.jupyter.org/github/ernestyalumni/Gravite/blob/master/EMsage.ipynb>
    [ipynb 
   
<https://raw.githubusercontent.com/ernestyalumni/Gravite/master/EMsage.ipynb>] 
   (Electromagnetic field 2-form from E and B, Maxwell equations, conserved 
   current, Lorentz force, Poynting vector)


but I get the error

AttributeError: 'DiffFormFreeModule_with_category.element_class' object has 
no attribute 'exterior_der'
Here's a MWE
M = Manifold(4, 'M', latex_name=r'\mathcal{M}', structure='Lorentzian') 
BL.<t,x,y,z> = M.chart(r"t x y z") 
var('E_x E_y E_z')
dt, dx, dy, dz = BL.coframe()[:]
E = E_x * dx + E_y * dy + E_z * dzE.set_name('E')
(E.wedge(BL.coframe()[0])).exterior_der()

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